Engineering metals such as aluminum alloys, titanium alloys, steels, and superalloys, are processed into sheet by melting the desired composition, casting the melt into ingots, hot rolling the ingots to slabs, and subsequently rolling the slabs into sheet form. Intermediate and final annealing operations can be performed on the rolled sheet to recrystallize the microstructure or obtain various properties such as improved ductility. In rolling, a squeezing type of deformation is accomplished by using two work rolls rotating in opposite directions. The principal advantage of rolling lies in its ability to produce desired shapes from relatively large pieces of metals at very high speeds in a somewhat continuous manner. Slabs are generally rolled at temperatures above the recrystallization temperature of the metal, or hot forming range, where large reductions in thickness are possible with moderate forming pressures. Smaller reductions can be made by cold rolling, forming below the temperature the metal will recrystallize, to maintain close thickness tolerances.
The multiple deformation forming processes in sheet rolling can produce a preferred crystal orientation or texture in the sheet. Crystals in certain orientations are more resistant to deformation than are other crystals. These deformation resistant oriented crystals tend to rotate during deformation thereby producing a preferred orientation. During recrystallization, preferred orientations result from the preferential nucleation and growth of grains of certain orientations.
Superalloys are difficult to deform and easy to crack during deformation. Since the superalloys were designed to resist deformation at high temperatures, it is not surprising that they are very difficult to hot work; the alloys having limited ductility and high flow stress. Furthermore, additional alloying elements which improved service qualities in the superalloy, usually decrease the ability to work or deform the alloy into a desired form. As a result, primary or slab rolling of superalloy sheet is usually performed at temperatures near the melting point of the alloy on rugged, powerful mills built to withstand the high stresses encountered in the working of superalloys, and fast handling is mandatory to minimize edge cracking. The superalloys have narrow working temperature ranges, and are often rolled in packs or layers, that are sometimes encased in a steel envelope, to minimize heat loss to the relatively cold rolls upon deformation. The narrow working temperature range makes the rolling labor intensive, and many intermediate reheating steps are required. Some of the superalloys that are commercially available in a sheet form are Hastelloy alloy X, IN-600, IN-718, IN-625, Rene, 41, and Waspaloy.
A combination of properties such as strength, formability, and weldability are desired in superalloy sheet, and the desired combination of properties dictate many aspects of the extensive thermomechanical processing required to form the sheet. However, equipment limitations may prevent performance of the required thermomechanical processing so that some desired properties may not be obtainable in sheets formed from some of the superalloy compositions. Because some sets of properties have not been attainable in cast alloy materials, resort is sometimes had to the preparation of parts by powder metallurgy techniques. However, one of the limitations which attends the use of powder metallurgy techniques in preparing moving parts for jet engines is that of the purity of the powder. If the powder contains impurities such as a speck of ceramic or oxide, the place where that speck occurs in the moving part becomes a latent weak spot where a crack may initiate. Some of the superalloy compositions that are prepared by powder metallurgy techniques are shown below in Table I.
TABLE 1 ______________________________________ Superalloy Compositions In Weight Percent Unitemp Astroloy Rene95 AF2-1DA IN100 ______________________________________ Ni Bal. Bal. Bal. Bal. Cr 15 13 12 10 Co 17 8 10 15 Mo 5.25 3.5 2.75 3 W 3.5 6.5 Nb 3.5 4.6 5.5 Ta 1.5 Al 4 3.5 4.6 5.5 Ti 3.5 2.5 2.8 4.7 C 0.06 0.06 0.04 0.05 B 0.03 0.01 0.02 0.014 Zr 0.05 0.06 V 0.09 ______________________________________
It is an object of this invention to provide a simplified method of forming metal into sheet.
It is another object of this invention to provide a method for forming sheets from metal compositions that are difficult to deform.
It is another object of this invention to provide a method for forming sheet having a substantially isotropic and fine crystal or grain structure .